Compressed n2 for energy storage

ABSTRACT

This disclosure describes a power system that includes a first compressor with an air inlet and a compressed air outlet; a nitrogen separator coupled to the compressed air outlet, the nitrogen separator comprising a nitrogen concentrate outlet and a byproduct outlet; a second compressor coupled to the nitrogen concentrate outlet, the second compressor having a high pressure outlet for supplying high pressure concentrated nitrogen to an underground storage; and a turbine generator with an inlet for high pressure concentrated nitrogen for coupling to an underground storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 62/596,240 filed Dec. 8, 2017, which is incorporated herein byreference.

FIELD

This application generally relates to electrical energy storage methods,and particularly to converting electrical energy to pressure energy, andvice versa, using compressed gas, such as nitrogen gas, for energystorage.

BACKGROUND

Electrical power storage is a great challenge facing the energyindustry. While chemical energy sources derive power from chemicalbonds, which can be stored as a liquid in a tank, renewable energysources such as wind and solar power cannot be conveniently andefficiently stored to meet high demand in the future. For example, peakproduction periods for renewable energy sources may not coincide withpeak demand periods for electric power. To balance supply and demand atpower generation units in the absence of storage, the units must bestarted and stopped as demand fluctuates. Such programs increasewear-and-tear on production facilities, and require predictivetechnologies to maintain power supply as loads increase.

It has been estimated that the world needs 40 TWh of battery storage toallow for 45% of power to be generated from renewable sources by 2030.Lithium ion battery technology is progressing to provide mass storage ofelectrical energy for years to come. However, demand is projected tooutstrip supply in the intermediate term. Lithium demand today is about32,000 metric tons per year, and growing. Global lithium reserves havebeen estimated at 13.5 million metric tons, with Chile and Chinatogether holding about 82% of global reserves. The price of lithium hasalready tripled since late 2015 due to surging demand for electricvehicles.

Pumped hydropower is a known technology that relies on pumping a fluid,such as water, from a low gravitational potential to a highergravitational potential, and then reclaiming that potential energylater. Such systems are typically predicated on large, usually naturallyoccurring, volumes of water nearby each other, and expensive pumping andrecovery infrastructure to deliver relatively meager returns in powerstorage. Compressed air storage technologies are also known, butcontainment suitable for compressed air, with its oxygen and moisturecontent is relatively limited so deployment of such technology willrequire construction of suitable containment.

With lithium supplies already sounding a warning klaxon regarding futuresupply of lithium battery energy storage, and other known technologiesfacing limits elsewhere, there is a need for new very large energystorage solutions.

SUMMARY

In one embodiment, this disclosure describes a power system thatincludes a first compressor with an air inlet and a compressed airoutlet; a nitrogen separator coupled to the compressed air outlet, thenitrogen separator comprising a nitrogen concentrate outlet and abyproduct outlet; a second compressor coupled to the nitrogenconcentrate outlet, the second compressor having a high pressure outletfor supplying high pressure concentrated nitrogen to an undergroundstorage; and a turbine generator with an inlet for high pressureconcentrated nitrogen for coupling to an underground storage.

Also described herein is a power storage system that includes a firstcompressor with an air inlet and a compressed air outlet; a nitrogenseparator coupled to the compressed air outlet, the nitrogen separatorcomprising a nitrogen concentrate outlet and a byproduct outlet; asecond compressor coupled to the nitrogen concentrate outlet, the secondcompressor having a high pressure outlet for supplying high pressureconcentrated nitrogen to an underground storage; a turbine generatorwith high pressure concentrated nitrogen inlet for coupling to theunderground storage and a gas outlet coupled to the nitrogen concentrateoutlet; and a thermal recovery unit coupled to the inlet of the turbinegenerator.

Also described herein is a power storage system coupled to a hydrocarbonreservoir. The power storage system includes a first compressor with anair inlet and a compressed air outlet; a liquids collector coupled tothe compressed air outlet; a nitrogen membrane unit coupled to theliquids collector, the nitrogen membrane unit comprising a nitrogenconcentrate outlet and a byproduct outlet; a second compressor coupledto the nitrogen concentrate outlet, the second compressor having a highpressure outlet for supplying high pressure concentrated nitrogen to anunderground storage; a turbine generator with high pressure concentratednitrogen inlet for coupling to the underground storage and a gas outletcoupled to the nitrogen concentrate outlet; and a thermal recovery unitcoupled to the inlet of the turbine generator.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present disclosure, briefly summarized above anddiscussed in greater detail below, can be understood by reference to theillustrative implementations of the disclosure depicted in the appendeddrawings. It is to be noted, however, that the appended drawingsillustrate only typical implementations of this disclosure and aretherefore not to be considered limiting of its scope, for the disclosuremay admit to other equally effective implementations.

FIG. 1A is a schematic configuration diagram of a power storage systemaccording to one embodiment.

FIG. 1B is a schematic configuration diagram of a compressor systemaccording to one embodiment.

FIG. 2 is a schematic configuration diagram of a power storage systemaccording to another embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The figures are not drawn to scale and may be simplifiedfor clarity. It is contemplated that elements and features of oneimplementation may be beneficially incorporated in other implementationswithout further recitation.

DETAILED DESCRIPTION

This disclosure describes methods and apparatus for power generation andenergy storage. FIG. 1A is a schematic configuration diagram of anenergy storage system 100 according to one embodiment. The energystorage system 100 includes an energy storage section 102 and an energyrecovery section 104, both coupled to a storage volume 106. The energystorage section 102 uses deployable power, such as electricity, topressurize a gas substantially depleted in oxygen, for exampleoxygen-depleted air or nitrogen gas, into the storage volume 106. Theenergy recovery section 104 recovers the energy stored in thepressurized gas to generate electricity. The storage volume 106 may beunderground or above ground, and may be natural or man-made. The energystorage systems described herein, including the energy storage system100, may be adapted for use with hydrocarbon reservoirs as undergroundpower storage. A gas substantially depleted in oxygen, such that theoxygen content of the gas is reduced below an explosive, or combustion,limit at elevated pressures can be used in the presence of hydrocarbonvapors without risk of explosion or fire.

The gas to be prepared for pressurized storage is provided to a firstcompressor 110 through an inlet 109. A filter 108, which may beself-cleaning, may be coupled to the inlet to remove particulates. Ahigh-flow air filter medium, such as one or more pleated cartridgefilters, or a self-cleaning pulse filter, or a gas turbine inlet filtermay be used for large installations. A preheater 103 may be used to heatthe material in the inlet to a convenient temperature for processing,for example a temperature above the dew point of the material. The firstcompressor 110 may raise the pressure of the gas to a pressure of 3-10barg, such as 4-7 barg, for example about 6 barg. The first compressor110 may raise the pressure of the gas to a pressure above 10 barg insome cases. The inlet pressure of the first compressor 110 may beatmospheric, or may be somewhat elevated to optimize operation of thefilter 108. The gas to be prepared for pressurized storage may beambient air at any naturally occurring temperature and humidity, or maybe pre-treated air (for example dried purified air). Additionally, thegas may be an air fraction or extract that is depleted in oxygen to anydegree, including pure or substantially pure nitrogen. In general thegas may be provided to the inlet of the first compressor 110 at anambient temperature from below 0° C. to above 40° C., for example fromartic temperatures such as −30° C. to desert temperatures such as 50°C., and a pressure from about 0.8 barg to about 4 barg, including anyambient pressure. In one embodiment, the first compressor 110 is anintegral gear compressor with discharge pressure ranging from 30 barg to110 barg. Other types of compressors, such as positive displacementcompressors, which may be rotary or piston driven, may be used as well.

The first compressor 110 may be a multi-stage compressor or amulti-compressor installation or system. The first compressor 110 has acapacity rated to the service. In one example, two to six stages ofcompression may be used in some cases for gas flow capacity of 1,200 to500,000 Nm³/hr. In general, the first compressor 110 may be sized to anyscale of operation, and indeed operations involving hydrocarbonreservoirs may utilize many cubic kilometers of storage volume with verylarge compression and recovery operations. When the first compressor 110comprises a plurality of individual compressors, the compressors may bemulti-stage integrally geared or multi-stage positive displacementcompressors, of reciprocating or rotary type. The compressors may bearranged in series, with the outlet of one compressor flowing to theinlet of another compressor, or in parallel, with the inlet gas dividedbetween the compressors by flow controls if desired, and the outlet gascombined into a single line.

The first compressor 110 may be equipped with air or water cooledinter-stage thermal units and liquid collection vessels. FIG. 1B is aschematic configuration diagram of a compressor system that may be usedas the compressor 110. The compressor 110, embodied in the compressorsystem of FIG. 1B, has two compression stages 110A and 110B. Followingeach of the compression stages 110A and 110B is a thermal recovery unit,111A and 111B respectively, to recover heat of compression. This heat ofcompression may be deployed in any convenient manner to improve theefficiency of the system 100. In addition, the outlet of eachcompression stage 110A and 110B may be equipped with a liquid collector,113A and 113B respectively, for collecting any condensed liquids. Thecompressor 110, as embodied by the compressor system of FIG. 1B, mayalso be equipped with a recycle stream, 117A and 117B respectively, foreach compression stage 110A and 110B, or a single recycle stream acrossall the compression stages. The recycle stream may feature a controlvalve, 121A and 121B respectively in FIG. 1B, to control compressorsurge. The compressor 110, embodied in the compressor system of FIG. 1B,may feature an inlet controller, 119A and 119B respectively, for eachcompression stage 110A and 110B (or only for one compression stage),such as a valve or inlet guide vane, to control gas flow volume and/orinlet pressure to the compressor 110.

The first compressor 110 has a compressed gas outlet 112, which may be acompressed air outlet when air is the gas that is compressed in thefirst compressor 110. The compressed gas may be flowed through a checkvalve 114 to prevent reverse flow into the compressor during start orafter shutdown of the first compressor 110. In the case where multiplecompressors are used, each individual compressor may use a check valvein the outlet of the compressor, whether the compressors are arranged inseries or in parallel. The compressed gas may also be flowed through athermal conditioner 115 to adjust the temperature of the compressed gasfor subsequent processing. Heat recovered from the thermal conditioner115 may be integrated to the preheater 103 using a thermal balancingloop 117 that circulates a thermal medium between the exchangers 115 and113. The compressed gas outlet 112 may be coupled to a liquids remover116, for example a liquids collection drum or a dessicator, for handlingany condensate arising from the first compression stage. A liquid stream118 may be withdrawn from the liquids remover 116 at a low point on theliquids remover 116. The liquid stream 118 may be released to theenvironment, if appropriate, through a safe environmental sewer system,or the liquid stream 118 may be treated before release or used for otherprocessing. The liquid stream 118, which may be condensate from moistair in some cases, may contain water or other readily condensablesubstances.

A liquids-relieved compressed gas 120 leaves the liquids remover 116 andflows to a nitrogen separator 124. The nitrogen separator 124 may be amembrane separator, a permeation separator such as a PSA unit, or athermodynamic separator such a cryogenic distillation unit or othercryogenic separator. A membrane separator, such as a hollow fibermembrane separator, may be used to separate oxygen from other gases suchas nitrogen and argon with acceptable efficiency. Multiple stages offiltration, for example 2-10 stages of filtration, can be used toincrease purity of the filtered nitrogen. Membrane nitrogen generatorsavailable from Schlumberger Ltd., of Houston, Tex., may be used as thenitrogen separator 124.

The nitrogen separator 124 has a nitrogen concentrate outlet 126 and abyproduct outlet 128. The nitrogen concentrate outlet 126 may beessentially pure nitrogen, or may be 90% or more nitrogen by volume, forexample 95% or 99.5% pure nitrogen by volume. The byproduct outlet 126may be a mixture of impurities, or may constitute more than one impuritystream. For example, in the case of air, such as ambient air or airpurified in any way, or in the case of an air-like compressed gas, theimpurity stream may include oxygen, CO₂, argon, helium, and other minorair components. For example, the nitrogen separator 124 may beconfigured to yield a substantially pure oxygen stream, which may beused for other processing.

The nitrogen concentrate outlet 126 is routed to a second compressor130, which boosts the pressure of the nitrogen concentrate. In one case,the second compressor 130 boosts the pressure of the nitrogenconcentrate to a pressure of 50-100 bar, for example about 80 barg. Inother cases, the first compressor 110 can pressurize the compressed gasoutlet 112 to a much higher pressure, for example up to 120 barg, toprovide higher pressure for membrane separation. The second compressor130 can then be used to recover pressure in the nitrogen concentratefollowing membrane separation.

The second compressor 130 may be, or use, the same type or types ofcompressors, optionally with inter-stage thermal recovery, as the firstcompressor 110, and may be configured as a multi-stage compressor ormultiple compressors arranged in series or parallel, as described above.In the event the first compressor 110 removes substantially allcondensables from the nitrogen concentrate outlet 126, liquid collectorsmay not be needed in the second compressor 130. In one embodiment, thesecond compressor 130 includes up to six compressors or compressionstages for a flow capacity of 500 Nm³/hr to 85,000 Nm³/hr. The secondcompressor 130 is generally scaled to the size of the installation, andmay be any size needed for the required duty. The second compressor 130may also be equipped with the same volume and safety control system asthe first compressor 110. The second compressor 130 has a high pressureoutlet 132 for supplying high pressure concentrated nitrogen to astorage volume 106, which may be an underground storage. As noted above,the storage volume 106 may be a hydrocarbon reservoir, which may bedepleted, or other natural reservoir or manmade underground reservoir.

Heat of compression may be recovered from the high pressure outlet 132using a compression thermal recovery heat exchanger 133. The highpressure concentrated nitrogen is cooled by the compression thermalrecover heat exchanger 133 to a temperature suitable for storage in thestorage volume 106, for example 20-50° C. Recovered heat of compressionmay be integrated to the preheater 103, if desired, using the thermalbalancing loop 117 further connected between the exchangers 133 and 113.

The high pressure outlet 132 may be routed to the storage volume 106through a check valve 134 and a pressurized gas control valve 136 via astorage line 138. If the storage volume 106 is a depleted hydrocarbonreservoir, the storage line 138 may be coupled to the reservoirwellhead. The pressures described above are generally suitable for theclass 600 piping commonly used for hydrocarbon wellheads. Removingoxygen and water from the gas lowers, or substantially prevents, thepossibility of corrosion in the piping system.

The energy recovery section 104 includes an energy recovery expander 156with a high pressure concentrated nitrogen inlet 140 that couples to thestorage volume 106. The energy recovery expander 156 is coupled to agenerator 158, the two forming a turbine generator. Gas from the storagevolume 106 is released to the inlet of the energy recovery expander 156,powering the generator to produce electricity, which can be provided tothe external power grid. In this way, the energy recovery section 104converts pressure in the stored gas into electricity, recovering thepower originally stored in the pressurized gas. A pressure sensor 142may be coupled to the high pressure concentrated nitrogen inlet 140 toallow monitoring of the pressure of the storage volume 106. Thecontroller 170 may be coupled to the pressure sensor 142 to monitor thepressure of the storage volume 106 utilizing the volume control systemof each of the compressors 110 and/or 130.

A thermal conditioner 144 may be coupled to the high pressure nitrogeninlet 140 to recover any excess heat the released gas may be carrying,or to utilize recovered heat of compression, to adjust the thermalcondition of the gas prior to entering the power recovery expander 156.If attractive and available from a local waste heat recovery system,external thermal heat may be added to the thermal conditioner 144 toenhance or increase the power generation of the power recovery expander156. Alternately, if attractive, excess heat in the gas of the highpressure nitrogen inlet 140 may be recovered in the thermal conditioner144 and returned to the preheater 103, depending on the specific systemconfiguration. A bypass 146 may be provided around the thermalconditioner 144 to provide a method of thermal control. A thermal sensor150, such as a temperature sensor, may be provided in the high pressurenitrogen inlet 140 downstream of the bypass 146 to allow monitoring ofthe temperature of the released gas. The thermal sensor 150 may be athermostat. The controller 170 may be coupled to the thermal sensor 150and the bypass control valve 148, and may adjust the bypass flow bymanipulating the bypass control valve 148 to control the temperature ofthe released gas at the thermal sensor 150.

If necessary to avoid charging substantial liquids to the energyrecovery expander 156, a phase separator 155 may be coupled to the highpressure nitrogen inlet 140 to the power recovery expander 156. Thephase separator 155 may be a membrane separator, a wire mesh structure,a distillation apparatus, or the like. A liquid stream 157 may berecovered from the phase separator 155 and routed to any convenient use.For example, hydrocarbons recovered from the phase separator 155 may beused for fuel or recovered into any usable product stream.

A release pressure control valve 152 controls inlet pressure at theenergy recovery expander 156. A pressure sensor 154 may be coupled tothe turbine inlet to allow monitoring of turbine inlet pressure. Thecontroller 170 may be coupled to the pressure sensor 154 to monitorturbine inlet pressure and may adjust the release pressure control valve152 to control the turbine inlet pressure. To reduce the turbine inletpressure below the pressure in the storage volume 106, the controller170 will manipulate the release pressure control valve 152 to throttlethe flow and create a pressure drop. The release gas flow rate can beindependently controlled using the vent control valve 162. To increaseturbine inlet pressure at constant flow rate, the release pressurecontrol valve 152 is opened incrementally while the vent control valve162 is closed incrementally. To decrease turbine inlet pressure atconstant flow rate, the release pressure control valve 152 is closedincrementally while the vent control valve 162 is opened incrementally.

The released gas drives the power recovery expander 156, which powersthe generator 158. The released gas loses pressure in the process andexits the energy recovery expander 156 through a gas outlet 160. Thepressure in the turbine outlet 160 will vary with release gas flow rate.Above the operable threshold of the turbine, pressure in the turbineoutlet will rise as gas flow increases. The gas outlet 160 is coupled toa vent 166 by a vent silencer 164, and a vent control valve 162, orflare system, can be used to control rate of gas venting and/or rate ofgas release from the storage volume 106. Vent gas bearing hydrocarboncould also be used as a fuel source or as an injection gas in oil andgas drilling. The power generator system (i.e. power recovery expander156 and generator 158) may be a commercial generating unit. The turbinemay be a radial inlet turbine, and the generator may be a commercialgenerator set. Suitable system will have complete electricity controlsuitable for interconnecting with a power grid system.

The controller 170 is operatively coupled to the first compressor 110,the second compressor 130, the pressurized gas control valve 136, thepressure sensor 142, the bypass control valve 148, the thermal sensor150, the release gas control valve 152, and the vent control valve 162.The controller monitors pressures, temperatures, and flow ratesthroughout the system 100, driving the system 100 to convert, forexample, grid electricity into pressure of the storage gas.

FIG. 2 is a schematic configuration diagram of a power storage system200 according to another embodiment. The power storage system 200includes substantially the same power storage section 102 and energyrecovery section 104 as the power storage system 100 of FIG. 1A, butalso includes a gas recycle section 201. The gas recycle section 201couples the outlet of the energy recovery expander 156 to the nitrogenconcentrate outlet 126. The gas recycle section 201 includes a recyclegas line 204 coupled to the vent line 166 upstream of the vent silencer164 and to the nitrogen concentrate outlet 126. A recycle gas controlvalve 208 may be provided in the recycle gas line 204 to provide controlover the recycle gas rate.

A gas storage 206 may be included to decouple flow from the vent line166 somewhat from flow into the nitrogen concentrate outlet 126. The gasstorage may be naturally occurring or man-made. For example, the storagevolume 106 and the gas storage 206 may both be depleted hydrocarbonreservoirs. Alternately, the storage volume 106 may be a hydrocarbonreservoir and the gas storage 206 may be a drum, which may beunderground, natural or manmade, or above ground. The gas storage 206may, in some cases, function to recompress the outlet gas from theenergy recovery expander 156, instead of venting or flaring. In suchcases, the pressure of the reservoir 206 is controlled to meet orslightly exceed the inlet pressure of the second compressor 130. The gasstorage 206 may, in some cases, also function as a liquids collector inthe event that gases from the storage volume 106 are condensed by thethermal conditioner 144. A manmade liquids collector may alternately beprovided coupled directly to the thermal conditioner 144.

A gas reconditioner 210 may be included along the recycle gas line 204to remove any gases not suitable for recycling into the power storagesection 102, such as hydrocarbon gases and/or sulfur-bearing gases. Thegas reconditioner 210 may also adjust the temperature and pressure ofthe recycle gas in the recycle gas line 204. The gas reconditioner 210may include a gas separator such as a distillation apparatus, anabsorption apparatus, a stripping apparatus, a combustion apparatus, oranother suitable gas separator. In some cases, the gas reconditioner 210may be a membrane nitrogen generator. The gas reconditioner 210 is shownin FIG. 2 between the recycle gas control valve 208 and the junction ofthe recycle gas line 204 and the nitrogen concentrate outlet 126, butthe gas reconditioner 210 may be anywhere along the recycle gas line204. For example, the gas reconditioner 210 may be positioned betweenthe pressure sensor 154 and the energy recovery expander 156, as shownin phantom in FIG. 2, or between the thermal conditioner 144 and therelease gas control valve 152, as shown in FIG. 1, to remove anymaterials, such as liquids, that might be incompatible with operation ofthe power recovery expander 156. The gas storage 206 is shown betweenthe junction of the recycle gas line 204 and the vent line 166 and therecycle gas control valve 208, but the gas storage 206 may also beanywhere along the recycle gas line 204.

The controller 170 can be coupled to the recycle gas control valve 208to control recycle gas flow rate. In one implementation, the controllermay control a ratio of fresh gas flow to recycle gas flow, which may becalled a makeup gas ratio. In some cases the ratio may be zero, at leasttemporarily until leakage within the storage volume 106 (i.e. leakageout of the hydrocarbon reservoir through geologic structures) reducespressure in the storage volume 106 to an undesirably low level, asdetected by the pressure sensor 142 and monitored by the controller 170.The controller 170 can then increase the ratio to add fresh gas to thesystem and raise the pressure of the storage volume 106 to a nominallevel. In the absence of a recycle gas reconditioner such as thereconditioner 210, the controller 170 may adjust the makeup gas ratio tocontrol or reduce impurities in the gas extracted from the storagevolume 106, such as hydrocarbons. As the gas is recycled through thestorage volume 106, such impurities may increase to an undesirablelevel. At that time, the controller 170 can increase the makeup gasratio to bring fresh gas into the system. Flow out the vent line 166will increase accordingly, carrying impurities out of the system withvent gas.

The apparatus described above, the power storage system 100 and thepower storage system 200, can be used to practice a method of powerstorage and recovery. The first and second compressors 110 and 130transform deployable power, such as electricity, into pressure in thegas stored in the storage volume 106. When power is readily availableand/or prices for power are attractive, the power storage section 102can be operated to transform electric power from the power grid intopressurized gas in the storage volume 106. Gas byproducts, if any, maybe sold or upgraded to provide additional incentive for operating thepower storage section 102. The systems 100 and 200 may be operated inthis manner until the storage volume 106 reaches a pressure limit oruntil consumption of power in the power storage section 102 is no longerattractive. The controller 170 can monitor power price and availability,and pressure in the storage volume using the pressure sensor 142, andwhen a stop condition is reached the controller 170 can signal thecompressors 110 and 130 and the control valve 136 to shut down, or idle,in an orderly fashion.

When power is in short supply or prices for power are high, the energyrecovery section 104 can be operated to transform pressure of thepressurized gas stored in the storage volume 106 into electricity. Thecontrol valve 152 can be opened, allowing pressurized gas to flow intothe energy recovery expander 156 to drive the generator 158 to makeelectric power, which can be delivered into the power grid. As notedabove, the controller 170 can monitor power price and availability, andwhen a start condition is reached the controller 170 can signal thecontrol valve 152 to open. The systems 100 and 200 may be operated inthis manner until the storage volume 106 empties or pressure in thestorage volume 106 drops below an operable threshold, or until recoveryof power in the energy recovery section 104 is no longer attractive.

Thus, the power storage section 102 can be operated and the energyrecovery section 104 idle at a first power availability, and the powerstorage section 102 can be idle and the energy recovery section 104operated at a second power availability lower than the first poweravailability. Alternately, the power storage section 102 can be operatedand the energy recovery section 104 idle at a first power price, and thepower storage section 102 can be idle and the energy recovery section104 operated at a second power price higher than the first power price.In some embodiments, both the power storage section 102 and the energyrecovery section 104 can be operated at a third power availabilitybetween the first and second power availability, or at a third powerprice between the first and second power price.

In operating the power storage section 102, conventional air, forexample ambient air, may be used as the starting gas, or other gases maybe used. For example, pure nitrogen could be charged directly to thesecond compressor 130, air that has been modified in some way could beused as the starting gas, or specific mixtures of gases that are not aircould be used as the starting gas. For example, air that has been driedcould be used as the starting gas, air that has been concentrated innitrogen to any extent could be used, a mixture of nitrogen with any ofhelium, argon, or other inert or substantially inert gas could be used.In some cases, if the storage volume is a hydrocarbon reservoir that nolonger produces, a gas that can extract hydrocarbon vapors from theresidue, for example CO₂, hydrogen, or another extraction gas may beincluded in the pressurized gas. When such gas is released through theenergy recovery expander 156 and exits through the vent 166, the gas maybe routed to a hydrocarbon recovery plant for extraction of thehydrocarbons. In the apparatus 200, the gas reconditioner 210

The start and stop conditions may overlap in ways that have the powerstorage section 102 and the energy recovery section 104 operating at thesame time. Under such conditions, the controller 170 may control bothsections to maintain a substantially constant pressure in the storagevolume 106, should such operating mode become attractive. The controller170 may slow the gas release rate by incrementally closing the ventcontrol valve 162 to allow pressure to build in the storage volume 106without consuming additional power in the compressors 110 and 130, orthe controller may add power to the compressors to add gas to thestorage volume 106, if it is attractive to do so. The controller 170 mayreduce pressure in the storage volume 106 by reversing such procedures.

Using the power storage system 200, pressurized gas released from thestorage volume 106 can be reused by routing depressurized gas exitingthe energy recovery expander 156 from the vent line 166 into the recyclesystem. By recycling released gas, power input to the first compressor110 can be reduced. In some cases, the storage volume 106 may haveliquid in it while being used for pressurized gas storage. If the liquidis hydrocarbon, the pressurized gas may volatilize some of thehydrocarbon. Recycling the released gas affords the opportunity torecover those hydrocarbons. In some embodiments, a hydrocarbon analyzermay be included in the gas reconditioner 210 and coupled to thecontroller 170 so the controller 170 can operate the gas reconditioner210 to optimize the gas composition for power storage as well as liquidsextraction. For example, using a gas with a low level of hydrocarbon mayfacilitate geologic extraction in the storage volume 106 while allowingfor liquids venting.

The power storage systems 100 and 200 described herein may be deployedas substantially fixed installations, or as mobile installations. Forexample, all or part of the systems 100 and 200 may be configured onskids or trailers to allow deployment in remote areas where a naturallyoccurring storage volume may be found. Systems such as those describedherein, from skid sized systems up to very large power plants, can beused with hydrocarbon reservoirs which can be very large. Suchreservoirs can provide steady flows of gas at pressure for months afterbeing fully pressurized. Such facilities promise to deliver gigajoulequantities of energy over long durations.

While the foregoing is directed to implementations of the presentdisclosure, other and further implementations of the disclosure may bedevised without departing from the basic scope thereof.

1. A power system, comprising: a first compressor with a gas inlet and acompressed gas outlet; a nitrogen separator coupled to the compressedgas outlet, the nitrogen separator comprising a nitrogen concentrateoutlet and a byproduct outlet; a second compressor coupled to thenitrogen concentrate outlet, the second compressor having a highpressure outlet for supplying high pressure concentrated nitrogen tostorage; and an expander and generator with a high pressure nitrogeninlet for coupling to storage.
 2. The power system of claim 1, whereinthe nitrogen separator is a nitrogen membrane unit, a nitrogengenerator, or a cryogenic separator.
 3. The power system of claim 1,wherein the first and second compressors are each multi-stagecompressors with one or more thermal recovery units.
 4. The power systemof claim 1, further comprising a pressure sensor coupled to the inlet ofthe turbine generator, a control valve coupled to the inlet of theturbine generator, and a pressure controller coupled to the pressuresensor and the control valve.
 5. The power system of claim 1, wherein agas outlet of the turbine generator is coupled to the nitrogenconcentrate outlet.
 6. The power system of claim 5, further comprising agas storage between the gas outlet of the turbine generator and thenitrogen concentrate outlet.
 7. The power system of claim 1, furthercomprising a thermal conditioner coupled to the inlet of the turbinegenerator.
 8. The power system of claim 7, further comprising a bypassaround the thermal conditioner, a control valve in the bypass, athermostat coupled to the inlet of the turbine generator between thethermal conditioner and the turbine generator, and a controller coupledto the thermostat and the control valve in the bypass.
 9. The powersystem of claim 1, further comprising a liquids collector coupled to anoutlet of the turbine generator.
 10. A power storage system, comprising:a first compressor with a gas inlet and a compressed gas outlet; anitrogen separator coupled to the compressed gas outlet, the nitrogenseparator comprising a nitrogen concentrate outlet and a byproductoutlet; a second compressor coupled to the nitrogen concentrate outlet,the second compressor having a high pressure outlet for supplying highpressure concentrated nitrogen to a storage volume; and a turbinegenerator with an inlet for coupling to the storage volume and a gasoutlet coupled to the nitrogen concentrate outlet.
 11. The power storagesystem of claim 10, wherein the nitrogen separator is a nitrogenmembrane unit, a nitrogen generator, or a cryogenic separator.
 12. Thepower storage system of claim 10, wherein each of the first compressorand the second compressor has a surge control recycle line and inletcontroller.
 13. The power storage system of claim 10, further comprisinga pressure sensor coupled to the inlet of the turbine generator, acontrol valve coupled to the inlet of the turbine generator, and apressure controller coupled to the pressure sensor and the controlvalve.
 14. The power storage system of claim 13, further comprising agas storage between the gas outlet of the turbine generator and thenitrogen concentrate outlet.
 15. The power storage system of claim 11,further comprising a thermal conditioner coupled to the inlet of theturbine generator, a bypass around the thermal conditioner, a controlvalve in the bypass, a thermostat coupled to the inlet of the turbinegenerator between the thermal conditioner and the turbine generator, anda controller coupled to the thermostat and the control valve in thebypass.
 16. The power storage system of claim 11, wherein the storagevolume is a hydrocarbon reservoir, the high pressure outlet is coupledto the hydrocarbon reservoir, and the inlet of the turbine generator iscoupled to the hydrocarbon reservoir.
 17. A method, comprising:producing a gas stream of at least 90% nitrogen by volume; pressurizingthe gas stream using a compressor; injecting the pressurized gas streaminto a storage volume; releasing gas from the storage volume to aturbine generator; and generating electricity using the turbinegenerator.
 18. The method of claim 17, wherein the storage volume is ahydrocarbon reservoir.
 19. The method of claim 18, further comprisingrecycling the released gas to the compressor.
 20. The method of claim19, further comprising conditioning the recycled gas.